The explosive growth in mobile electronic devices such as smartphones and tablets creates an increasing need in the art for compact and efficient switching power converters so that users may recharge these devices. A flyback switching power converter is typically provided with a mobile device as its transformer provides safe isolation from AC household current. This isolation introduces a problem in that the power switching occurs at the primary side of the transformer but the load is on the secondary side. The power switching modulation for a flyback converter requires knowledge of the output voltage on the secondary side of the transformer. Such feedback can be obtained through opto-isolators bridging from the secondary side to the primary side but this adds to cost and control complexity. Thus, primary-only feedback techniques have been developed that use the reflected voltage on the primary side of the transformer in each switching cycle.
In a switching cycle for a flyback converter, the secondary current (the current in the secondary winding of the transformer) pulses high after the primary-side power switch is cycled off. The secondary current then ramps down to zero as power is delivered to the load. The delay between the power switch off time and the secondary current ramping to zero is denoted as the transformer reset time (Trst). The reflected voltage on the primary winding at the transformer reset time is proportional to the output voltage because there is no diode drop voltage on the secondary side as the secondary current has ceased flowing. The reflected voltage at the transformer reset time is thus directly proportional to the output voltage based upon the turn ratio in the transformer and other factors. Primary-only feedback techniques use this reflected voltage to efficiently modulate the power switching and thus modulate the output voltage.
One issue, however, with primary-only feedback occurs during low-load or no-load periods of operation. The controller in the flyback converter detects this lack of activity and stops cycling the power switch accordingly so that the secondary side is not driven out of regulation. The resulting lack of pulsing is satisfactory so long as the load remains dormant. But should the load again be applied, the controller has no way of detecting this without a secondary current pulse being generated to produce reflected voltage on the primary side (for example, as sensed through a primary-side auxiliary winding).
To solve this problem in primary-only feedback architectures, an activity detector is provided on the secondary side that is configured to generate a secondary current pulse in response to the application of the load despite the power switch continuing to be dormant. An example of an activity detector is provided by commonly-assigned U.S. application Ser. No. 14/340,482, (the '482 application) filed Jul. 24, 2014, the contents of which are hereby incorporated by reference in their entirety. The secondary-side activity detector detects the termination of a secondary winding current pulse as generated conventionally from a cycling of the primary-side power switch. As discussed above, the termination of the secondary winding current pulse occurs at the transformer reset time. After this transformer reset time, the voltage across the primary-side auxiliary winding will oscillate due to the resonant circuit formed by the inductance of the transformer and the parasitic capacitance of the power switch. Since this oscillation could be interpreted by the controller as the application of a load (or occurrence of a fault condition), the secondary-side activity detector will not generate an activity pulse for a “blanking period” after the transformer reset time to allow the auxiliary winding oscillations to sufficiently subside. Upon the termination of the blanking period, the activity detector monitors the voltage drop across the rectifying diode on the secondary side to determine whether a load has been applied. With an applied load, the voltage across the rectifying diode changes as the load capacitor discharges. The activity detector detects this voltage difference across the rectifying diode and switches on a low-impedance current path that bypasses the rectifying diode. Advantageously, the activity detector may comprise a two-terminal device such as disclosed in the '482 application that couples to the cathode and anode of the rectifying diode. In other architectures, the activity detector may comprise a three-pin (or higher) device that raises manufacturing costs as compared to the use of a two-terminal activity detector. Should the activity detector detect a load-induced voltage change across the rectifying diode subsequent to the blanking period, it shorts the cathode and anode of the rectifying diode through its low-impedance alternative current path. This low-impedance current path enables the charged output capacitor on the secondary side of the transformer to send a pulse of secondary current that creates a reflected pulse on the primary-side auxiliary winding. The flyback controller is configured to detect this secondary current pulse. Since this secondary current pulse is not created by the pulsing of the power switch, the corresponding pulse in the reflected voltage is denoted herein as an “activity signal” to distinguish it from the reflected voltage obtained from a power switch cycle.
In response to detecting the activity signal, the flyback controller cycles the power switch pulse. The resulting reflected voltage from cycling of the power switch may then be used as is known in primary-only feedback techniques to directly monitor the output voltage so that it may be regulated accordingly. Although the generation of an activity signal is quite advantageous to address the lack of feedback information while the load is dormant or absent, the resulting flyback control suffers from several problems. For example, the flyback controller cannot receive an activity signal during the duration of the blanking period. The flyback controller thus is unable to respond to any sudden applications of a load during the blanking period. But this blanking period must be sufficiently long to encompass the voltage oscillations on the auxiliary winding that occur after the transformer reset time. In addition, the activity signal amplitude diminishes over time with respect to the blanking period termination. In other words, should activity be detected relatively shortly after the blanking period termination, the resulting activity signal as reflected onto the primary-side auxiliary winding will be relatively strong. In contrast, should activity be detected after a longer delay from the blanking period termination, the activity signal amplitude will be weaker. The snubber circuit associated with the primary winding may cause this reduction in activity signal amplitude with respect to the delay from the blanking period termination. The threshold used by the flyback converter to detect the presence of the activity signal should thus be relatively low so that the weaker activity signals from relatively long delays can be detected. But such a low threshold raises the possibility of responding to noise.
Accordingly, there is a need in the art for improved flyback control techniques to detect activity of the load following a period of reduced or dormant power switch cycling.